Myopia or short sightedness is a condition in which the far-point of the eye is less than infinite in distance from the eye. Thus a myopic eye can see objects clearly only within a finite distance, the limit of that far distance moving closer to the eye as the level of myopia increases. Advancing myopia is the result of the scleral ball of the eye elongating (axial myopia) so that the retina that lies against the posterior inner wall of the eye moves behind the eye's distance image focal point. In correcting this condition, a light diverging or “minus” lens must be used to move the focused light from in front of the retina backward to the retinal plane. The minus lens, whether spectacle lens, contact lens or other light diverging ophthalmic appliance, allows the myopic eye to regain the clarity of objects at an infinite distance.
Emmetropization is a process by which the eye continues to develop and grow from birth until full maturity when the process is discontinued. As the eye grows in size, it makes adjustments to both its optical component sizes and its refractive shapes. The adjustments are normally coordinated by chemical processes within the eye so that the eye converges toward the balanced condition known as emmetropia: does not need aids to see clearly at all distances. For reasons not fully understood, emmetropization does not always proceed as it should, leaving the eye myopic (and usually progressively so) or, less often, hyperopic.
It has been hypothesized that the development of the eye refractive state is driven by many interacting influences including the eye shape, retinal light patterns, and the nature of visual stimuli experienced. Additionally, individual differences in eye shape and its peripheral refractive condition that are integrated to initiate a chemical growth signal, all determine the direction of emmetropization. Therefore, we are able to assess certain clinically observable factors.
For close work and reading (in young healthy eyes), the mechanism of the crystalline lens and ciliary muscles increase the crystalline lens power to focus objects that are closer than infinity. The term “accommodation” is used to indicate this action of eye's internal optics to create a more focused image on the retina. Accommodation involves use of internal ciliary muscles and external muscles that further increases stress to the eyeball itself. Therefore, aspects of accommodation itself are believed to increase risk of myopic development. Accommodative lag is an ocular anomaly found usually in myopic subjects wherein the eyes lag behind the near focus of an object of regard such as reading print. Accommodative lag has been shown to be a risk factor in inducing myopia.
Myopia typically begins with certain discernable but often nebulous symptoms that are sometimes referred to as “prodromal symptoms of myopia” or “school myopia”. Frontal headache, transient blurred vision both near and far, and difficulty reading are symptoms of accommodative stress and beginning myopia in young school children. Nearwork-induced transient myopia (NITM) is found usually in older students and adults, but presents similarly. An overall descriptor of these two similar but separate conditions is the term pseudomyopia. Pseudomyopia is an intermittent, temporary shift in refraction of the eye toward myopia, in which the focusing of light in front of the retina is due to a transient spasm of the ciliary body or muscle group. This in turn causes a temporary increase in refractive power of the eye. A user perceived result is impaired distance vision. It is believed by most eye-care practitioners that NITM is another major risk factor that connects nearwork-related symptoms to the development of permanent myopia.
Inherently, myopia control involves relatively youthful eyes and, when designing or prescribing optical appliances such as contact lenses or spectacles, the particular issues and needs of the young must be addressed if myopia is to be successfully thwarted. Young users of corrective lenses have been found to demonstrate a high degree of sensitivity to any form of optical distraction. This has been illustrated by their difficulty in maintaining use of progressive power spectacle lenses with any great amount of power variation. Also, optical defects such as diffraction from optical discontinuities such as those in traditional bifocal lenses are a source of particular discomfort and prevent long-term use by many young users. Further, due to the extended periods of close work experienced by many school age youth, accommodation and accommodative stress is a source of discomfort that may sometimes limit use of corrective lenses by the young.
While many optical methods and devices have been proposed in the prior art for remedying myopia or myopia progression, all suffer from some aspect that prevents long-term use by the young. Unless the treatment method or device can be comfortably and reliably used by young persons, myopia control is not practicable. The result is that no real solution is available to address myopia. What is needed is an optical lens and method of treatment that counteracts the risk factors associated with myopia progression including accommodative stress, NITM and related pseudomyopia and at the same time addresses the particular needs of the young person so that a successful result can be obtained.